Influence of Compaction Length on Radial Melt Segregation in Torsionally Deformed Partially Molten Rocks

Abstract

To investigate the influence of compaction length on radial melt segregation during torsional shear deformation of partially molten rocks, experiments were performed on samples composed of olivine plus ∼7 vol.% of either an albite, alkali basalt, or lithium silicate melt. These three melts cover a range of three orders of magnitude in viscosity, yielding samples that vary by approximately two orders of magnitude in compaction length. Samples were deformed in torsion at 1,473 K and 300 MPa in constant strain rate experiments to outer‐radius shear strains of up to 14.3. Radial melt segregation occurred toward the axial center in all three types of samples that were sheared to γfalse(Rfalse)1em≥ 4. At the same strain, samples with the largest compaction length exhibited the highest segregation rate, while samples with intermediate and smallest compaction lengths exhibited similar segregation rates. The experimental observations qualitatively agree with previously published results from two‐phase flow theory for base‐state melt segregation with anisotropic viscosity; specifically, the segregation rate for radial melt segregation increases with increasing compaction length. However, quantitatively, the segregation rate in experiments is smaller than the rate predicted by simulations for the same compaction length. This discrepancy may, for example, reflect the difference in rheological behavior between that observed in our experiments (non‐Newtonian, dislocation‐accommodated creep) and that incorporated into the numerical models (Newtonian, diffusion‐accommodated creep). Our results thus provide a baseline for testing current and future models of two‐phase flow, particularly as applied to understanding melt migration, segregation, and extraction from Earth's deeper interior.

Radial Melt Segregation During Extrusion of Partially Molten Rocks

“…Furthermore, analysis of rotary shear of a partially molten material yielded melt segregation at a broader, sample scale (Takei & Holtzman, 2009), consistent with results from experiments (Gourlay et al, 2007). This broader-scale segregation has recently been reported in torsion experiments on partially molten aggregates of olivine plus basalt (Qi et al, 2015;Qi & Kohlstedt, 2018;Kohlstedt et al, 2014). Further theoretical analysis of melt segregation in a number of different deformation geometries including simple shear flow, parallel-plate Poiseuille flow, torsional flow , and pipe Poiseuille flow (Allwright & Katz, 2014) demonstrated that, based on two-phase flow theory with viscous anisotropy, broad scale melt segregation is expected for the latter three deformation geometries.…”

“…For compaction lengths smaller than the sample size, Allwright and Katz (2014) predicted that melt-rich sheets should extend inward from the channel wall inclined at a low angle and antithetic to the imposed flow direction, similar to those predicted and observed for general shear and torsional deformation geometries Holtzman et al, 2003;King et al, 2010;Kohlstedt et al, 2010;Qi et al, 2015;Qi & Kohlstedt, 2018). For compaction lengths smaller than the sample size, Allwright and Katz (2014) predicted that melt-rich sheets should extend inward from the channel wall inclined at a low angle and antithetic to the imposed flow direction, similar to those predicted and observed for general shear and torsional deformation geometries Holtzman et al, 2003;King et al, 2010;Kohlstedt et al, 2010;Qi et al, 2015;Qi & Kohlstedt, 2018).…”

“…where d is grain size, ϕ is melt fraction, r is the melt fraction exponent, and C is a constant. For compaction lengths smaller than the sample size, Allwright and Katz (2014) predicted that melt-rich sheets should extend inward from the channel wall inclined at a low angle and antithetic to the imposed flow direction, similar to those predicted and observed for general shear and torsional deformation geometries Holtzman et al, 2003;King et al, 2010;Kohlstedt et al, 2010;Qi et al, 2015;Qi & Kohlstedt, 2018). For compaction lengths larger than the sample size, only base-state segregation toward the wall of the channel was predicted (Allwright & Katz, 2014).…”

Does Io Have a Magma Ocean?

“…Melt distribution also depends on the confining pressure; at high pressures (>1 GPa, >200 km depth in Io), it was shown that melt distribution corresponds to a disequilibrium phenomenon that can be caused by the non-hydrostatic state of stress or the induced strain (e.g., Bai et al, 1997;Jin et al, 1994). Recently, it was also demonstrated that compaction length has a strong influence on melt migration and extraction processes in a partially molten rock undergoing deformation (Qi and Kohlstedt, 2018). Understanding the rheological properties and dynamics of a melt-bearing mantle would strongly benefit from viscosity measurements under high pressure and temperature conditions (e.g., Liebske et al, 2005;Tinker et al, 2004) on molten analogues of Io's mantle.…”

Influences of CO2 on the Microstructure in Sheared Olivine Aggregates

“…Deformation and melt transport, including melt migration and segregation, are strongly coupled in partially molten rocks, as evidenced by field observations [1], laboratory experiments [2], and theoretical analyses [3]. Significantly, melt spontaneously localizes into melt-enriched bands oriented ∼20° to the shear plane, antithetic to the shear direction in partially molten rocks deformed in general and torsional shear [4][5][6][7]. The formation of melt-enriched bands is predicted and modeled by the two-phase flow theory with anisotropic viscosity [3,[8][9].…”